1. Field of the Invention
The present invention relates to radiant energy detectors, and more particularly relates to uncooled background limited detectors and methods for detecting radiant energy utilizing uncooled background limited detectors.
2. Description of the Prior Art
The figure of merit for a radiant energy detector is considered to be the detector's sensitivity (i.e., detectivity). Detectivity, symbolically denoted as D*, is measured in cm-Hz.sup.1/2 /W. In relation to other detectors, a detector with a higher D* is able to detect smaller quantities of energy, in various forms, making the detector useful in a variety of applications. However, prior art radiant energy detectors seeking to achieve a high degree of sensitivity have required the use of costly cryogenic cooling systems associated with detecting smaller quantities of energy. For example, a prior art detector such as the mercury-cadmium-tellerium (HgCdTe) detector is required to be cooled with liquid nitrogen or mechanical coolers to a temperature of 80 degrees Kelvin in order to achieve a high degree of sensitivity. Such cooled detectors are not only expensive but also are limited in the duration of time that they can operate which, in turn, disadvantageously affects the detection capability of the detector. Likewise, such prior art cooled detectors are known to suffer reliability problems due to the maintenance cycle associated with mechanical coolers.
In addition, detector sensitivity and, thus, the performance of a radiant energy detector, is affected by whether or not the detector is background limited. Background limited performance (BLIP) is a desirable attribute for a detector to possess because such a BLIP detector has the best achievable sensitivity. Detectors that are not background limited suffer from the introduction of other noise, such as Johnson noise, which is a limiting factor in the performance of the detector.
There have been many efforts to develop high sensitivity uncooled detectors. Such prior art designs have included the combination of several individual detectors to form a focal plane array. There have been several notable developments in this area such as the microbolometer array developed by Loral/Honeywell and the ferroelectric array developed by Texas Instrument. These efforts have resulted in focal plane arrays having a D* of approximately 2.times.10.sup.8 cm-Hz.sup.1/2 /W to 4.times.10.sup.8 cm-Hz.sup.1/2 /W.
Attempts to achieve high sensitivity in single element detectors have been the object of work carried out by AIL Systems, Inc. of Deer Park, N.Y. AIL Systems, assignees of the present invention, has fabricated a metal-oxide-metal tunnel diode (MOMTD) on a 1000 .ANG. (angstrom) SiO.sub.2 membrane. Initial experiments and analyses indicate that a D* of 10.sup.9 was achievable for a MOMTD on a membrane for a detector bandwidth of 300 Hz. However, the limiting factor with respect to the detector sensitivity was found to be the thermal conductance associated with the contact leads of the MOMTD. This was due to the fact that thermal fluctuation associated with the thermal conductance of the contact leads was a source of noise which, in turn, severely degraded the performance of these prior art detectors.